1. Field of the Invention
The present invention is directed to electronic circuits for generating exponential functions. More specifically, but without limitation thereto, the present invention is directed to an electronic circuit for generating an output current that is an exponential function of a control current.
2. Description of the Prior Art
In many analog and radio frequency (RF) communications systems, for example, television and cellular telephony, the received signal often has a highly variable amplitude. This variation in amplitude is typically a result of transmission distance, fading, and various imperfections in the transmission medium. To prevent saturation of the receiver while maximizing the signal-to-noise ratio, the receiver gain is typically controlled by an automatic gain control circuit. The automatic gain control generally includes a variable gain amplifier (VGA) having a gain that is a function of a control voltage or current.
In one aspect of the present invention, a precision analog exponentiation circuit includes a first transistor coupled to a reference current for generating a voltage at the first transistor, a second transistor coupled to the first transistor for generating an output current, a variable current source coupled to the first transistor and the second transistor for generating a sum of the reference current and the output current in response to a feedback signal, and a feedback amplifier coupled to the first transistor for generating the feedback signal wherein the variable current source maintains the voltage at the first transistor substantially equal to a reference voltage so that the output current is substantially equal to an exponential function of a control voltage coupled to the first transistor and the second transistor.
In another aspect of the present invention, a method of generating a precision analog exponentiation function includes steps for generating a reference current through a first transistor to generate a voltage at the first transistor, generating an output current through a second transistor, generating a sum of the reference current and the output current by a variable current source in response to a feedback signal, and generating the feedback signal to maintain the voltage at the first transistor substantially equal to a reference voltage so that the output current is substantially equal to an exponential function of a control voltage coupled to the first transistor and the second transistor.